Crankcase lubricating oils typically contain additives to enhance various aspects of oil performance. Such additives are usually mixtures of several component additives, some of which may be oil soluble polymers or derivatised polymers. Typical of such polymeric additive components are ashless dispersants and viscosity modifiers.
Ashless dispersants maintain in suspension oil insolubles resulting from oxidation of the oil during wear or combustion. They are particularly advantageous for preventing the precipitation of sludge and the formation of varnish, particularly in gasoline engines.
Ashless dispersants comprise an oil soluble polymeric hydrocarbon backbone bearing one or more functional groups that are capable of associating with particles to be dispersed. Typically, the polymer backbone is functionalised by amine, alcohol, amide, or ester polar moieties, often via a bridging group. The ashless dispersant may be, for example, selected from oil soluble salts, esters, amino-esters, amides, imides, and oxazolines of long chain hydrocarbon substituted mono and dicarboxylic acids or their anhydrides; thiocarboxylate derivatives of long chain hydrocarbons; long chain aliphatic hydrocarbons having a polyamine attached directly thereto; and Mannich condensation products formed by condensing a long chain substituted phenol with formaldehyde and polyalkylene polyamine.
The oil soluble polymeric hydrocarbon backbone of these dispersants is typically derived from an olefin polymer or polyene, especially polymers comprising a major molar amount (i.e., greater than 50 mole %) of a C.sub.2 to C.sub.18 olefin (e.g., ethylene, propylene, butylene, isobutylene, pentene, octene-1, styrene), and typically a C.sub.2 to C.sub.5 olefin. The oil soluble polymeric hydrocarbon backbone may be a homopolymer (e.g., polypropylene or polyisobutylene) or a copolymer of two or more of such olefins (e.g., copolymers of ethylene and an alpha-olefin such as propylene or butylene, or copolymers of two different alpha-olefins). Other copolymers include those in which a minor molar amount of the copolymer monomers, for example, 1 to 10 mole %, is an .alpha.,.omega.-diene, such as a C.sub.3 to C.sub.22 non-conjugated diolefin (for example, a copolymer of isobutylene and butadiene, or a copolymer of ethylene, propylene and 1,4-hexadiene or 5-ethylidene-2-norbornene).
Viscosity modifiers (or viscosity index improvers) impart high and low temperature operability to a lubricating oil. Compounds used generally as viscosity modifiers include high molecular weight hydrocarbon polymers, including polyesters. Oil soluble viscosity modifying polymers generally have weight average molecular weights of from about 10,000 to 1,000,000, preferably 20,000 to 500,000, which may be determined by gel permeation chromatography or by light scattering.
Ashless viscosity modifiers that also function as dispersants are also known. In general, these dispersant viscosity modifiers are functionalised polymers (for example, copolymers of ethylene-propylene post grafted with an active monomer such as maleic anhydride) which are then derivatised with, for example, an alcohol or amine.
Additives comprising mixtures of ashless dispersants and viscosity modifiers are described in the art.
EP-A-307,132 discloses mixtures of two ashless dispersants each being a mono- or di-carboxylic acid-based derivative of a C.sub.2 to C.sub.10 monoolefin polymer. Mixtures of two dicarboxylic acid-based derivatives of polyisobutylene homopolymers are exemplified in Examples 6 and 7, in combination with an ethylene-propylene copolymer viscosity modifier. Improved diesel engine piston cleanliness is with these examples.
Improved ashless dispersants having enhanced sludge dispersion properties are disclosed in, for example, EP-A-440,505 and U.S. Pat. No. 5,266,223, being derived from ethylene-alpha olefin copolymers wherein at least about 30 percent of the polymer chains possess terminal vinylidene (i.e. ethenylidene) unsaturation. The combination of one specific group of improved dispersants having high number average molecular weight with other ashless dispersants such as polyalkenyl succinimides of C.sub.3 -C.sub.4 olefins and with viscosity modifiers is disclosed in EP-A-440,505.
U.S. Pat. No. 5,266,233 describes one low number average molecular weight class of these improved dispersants wherein an ethylene-propylene copolymer is functionalised by mono- or dicarboxylic acid moieties via an `ene` reaction or chlorination reaction. Mixtures of polyisobutene-based dispersants with 18 mole % of such improved dispersants are described as having useful viscometric properties. Such mixtures may be used with other conventional additive components, such as ethylene copolymer viscosity modifiers.